The present invention relates to a direct conversion receiver for use in radio systems.
It is known to use direct conversion in radio receivers which avoid the use of bulky and expensive bandpass IF filters. Also, the use of RF filters is considerably simplified and a complete receiver may be integrated onto a single chip.
In practice, one of the main obstacles to the successful implementation of direct conversion, particularly in the digital communications environment, is a requirement for automatic gain control (AGC). In a direct conversion receiver, AGC must be derived from baseband, which often makes it too slow for use with the bursted signals commonly used in modern digital systems.
Referring to FIGS. 1 and 2, FIG. 1 shows an arrangement which is often used in low data rate paging receivers and includes an antenna 2 connected to an input of a blocking filter 4, the output of which is connected to an input of an amplifier 6. The output of the amplifier is applied to an input of a mixer 8, 10 respectively, which receive at a second input thereof, an output from an oscillator 12. The mixer 8 receives a signal which is at 0.degree. phase from the oscillator 12, and the mixer 10 receives a signal which is at 90.degree. phase from the oscillator 12. The outputs of the mixers 8 and 10 are applied respectively to an input of a low pass filter 14, 16, the output of which is applied to limiters 18, 20 respectively. The output from the limiter 18 is an in-phase signal I, and the output from the limiter 20 is a quadrature phase signal Q. The circuit thus described does not require any AGC.
If the inputs of the receiver is a frequency shift keyed (FSK) signal, it may be represented vectorially as shown in FIG. 2. The left hand diagram of FIG. 2 shows that the input can have any phase angle, whereas the output, after hard limiting, the I and Q signals are quantized to any four possible phase states, as shown in the right hand diagram of FIG. 2.
In order to demodulate the FSK modulation, it is necessary to establish the direction of rotation of the vector. This is straight forward in paging systems where the modulation index is high, since the vector will rotate several cycles for each data bit. The limited I and Q output signals then become a burst of square waves at 90.degree. to each other, either leading or lagging, depending upon the direction of rotation. By comparing the limited I and Q signals in a phase sensitive detector (e.g. a D-Type flip-flop), the polarity of the phase difference, and hence modulation can be recovered.
However, in more spectrally efficient, low modulation index schemes such as Gaussian frequency shift keying (GFSK), the vector can rotate as little as 50.degree. per data bit. This means that the vector can remain entirely within one quadrant, so there is no change at the limiter outputs. In this case the data is not recoverable.